1. Field of the Invention
The present invention relates to an electrostatic chuck, and associated manufacturing process, for use in an etching apparatus for manufacturing semiconductors.
2. Description of the Related Art
In a semiconductor manufacturing process, etching processes are repeatedly performed together with insulating film formation, diffusion processes, and photolithographic processes. There are two types of etching processes: wet etching and dry etching. The dry etching process is implemented using a plasma etching apparatus as shown in FIG. 4. For example, with a semiconductor wafer W held on a chuck 12 in a processing chamber 11 of the etching apparatus, a reactive gas is introduced from an inlet 13 into the processing chamber 11 while high-frequency electric power 15 is applied between the chuck 12, which serves as a lower electrode, and an upper electrode 14 to generate a plasma in the processing chamber 11. Chemical reactions with radicals in the plasma and accelerated ions cause the semiconductor wafer W to be etched. More particularly, either the semiconductor wafer W itself or an insulating film (not shown) thereon is etched.
As mentioned above, during the dry etching process, the semiconductor wafer W is held on the chuck 12. In recent etching apparatuses, the chuck 12 has been specified to be an electrostatic chuck. Electrostatic chucks have demonstrated excellent characteristics in vacuum plasma processors.
The electrostatic chuck generates electrostatic forces for attracting material to the chuck. The electrostatic forces include two types: a Coulomb force and a Johnsen-Rahbek force.
Additionally, there are two types of electrostatic chucks: a unipolar type and a bipolar type. With respect to FIG. 5, the unipolar type of electrostatic chuck includes an anode 22 formed in a dielectric material 21. Also in the unipolar type, a cathode is defined by the apparatus such that a plasma electric potential is produced as shown in FIG. 5. With respect to FIG. 6, the bipolar type of electrostatic chuck includes both an anode 32 and a cathode 33 formed in a dielectric material 31.
FIG. 7 is an illustration showing a cross-sectional view of a conventional electrostatic chuck, in accordance with the prior art. The conventional electrostatic chuck includes an anodized aluminum film 42 disposed as a dielectric material on a surface of a disc-shaped aluminum electrode 41. The anodized aluminum film 42 has a thickness within a range extending from 50 μm to 60 μm. The semiconductor wafer W is placed on the anodized aluminum film 42. A cooling gas channel 43 is formed on the surface of the aluminum electrode 41 that is covered by the anodized aluminum film 42. The cooling gas channel 43 extends in a circumferential direction following a periphery of the aluminum electrode 41. A helium cooling gas is fed from gas feed orifices (not shown), penetrating the anodized aluminum film 42 and the aluminum electrode 41, to the cooling gas channel 43. The helium cooling gas flows into the cooling gas channel 43, fills the cooling gas channel 43, and then diffuses along the entire interface between the anodized aluminum film 42 and the semiconductor wafer W. The helium cooling gas diffusion occurs through fine gaps present along the interface between the anodized aluminum film 42 and the semiconductor wafer W. The fine gaps are defined by a rough surface of the anodized aluminum film 42. The helium gas diffusion serves to cool the semiconductor wafer W. In the dry etching process performed using the apparatus of FIG. 4, a temperature of the semiconductor wafer W can significantly affect the resulting etching characteristics. Use of the helium cooling gas as previously described serves to cool the semiconductor wafer W by as much as 30° C. to 60° C., thus improving the resulting etching characteristics, especially a uniformity characteristic.
The conventional electrostatic chuck as described above can be adversely affected through reaction product deposition. More specifically, reaction products present in the chamber 11 can adhere to a surface of the anodized aluminum film 42 after the semiconductor wafer W is removed from the chuck following the etching process, thus weakening the electrostatic attraction capability of the chuck in subsequent etching processes. Additionally, reaction products adhering to the surface of the anodized aluminum film 42 can increase gaps between the anodized aluminum film 42 and the semiconductor wafer W, causing leakage of the helium cooling gas from an outer peripheral edge of the aluminum electrode 41. Consequently, leakage of the helium cooling gas can cause the semiconductor wafer W to be insufficiently cooled, thus causing the etching characteristics to be adversely affected. Furthermore, during the etching process, the anodized aluminum film 42 can be deteriorated by reactive gases or ions which pass through end portions of the semiconductor wafer W, thus further weakening the electrostatic attraction capability of the chuck.
In response to the aforementioned problems, an electrostatic chuck has been developed that incorporates a ceramic layer as the dielectric material. FIG. 8 is an illustration showing an electrostatic chuck incorporating a ceramic dielectric layer, in accordance with the prior art. This electrostatic chuck of FIG. 8 has a ceramic layer 53 bonded onto a disc-shaped metal base plate 51 by means of an adhesive layer 52. A high-melting point electrode 54 is laid in the ceramic layer 53. In this arrangement, from the viewpoint of increasing electrostatic attraction, the electrode 54 is positioned near to the surface of the ceramic layer 53. In one example, the ceramic layer 53 is 1 mm thick and the electrode 54 is positioned 0.3 mm away from a top surface of the ceramic layer 53 and 0.7 mm away from a bottom surface of the ceramic layer 53. As with the electrostatic chuck of FIG. 7, the electrostatic chuck of FIG. 8 also includes a cooling gas channel 55 formed in the top surface of the ceramic layer 53 and extending in a circumferential direction following a periphery of the ceramic layer 53. The cooling gas channel 55 needs to have a certain depth considering a flow of the helium cooling gas. Therefore, if the cooling gas channel 55 is formed at a location overlying the electrode 54, the close proximity of the electrode 54 to the top surface of the ceramic layer 53 causes a distance between the bottom of the channel 55 and the electrode 54 to become short. If the distance between the bottom of the channel 55 and the electrode 54 becomes too short, the ceramic layer 53 spanning the short distance can have an insufficient dielectric strength. To avoid the insufficient dielectric strength issue, the electrostatic chuck of FIG. 8 has the channel 55 formed 1 mm to 2 mm within an outer periphery of the ceramic layer 53, and the electrode 54 formed within an outer boundary defined by the channel 55.
The electrostatic chuck of FIG. 8, however, is not without problems. More specifically, since the electrode 54 has a coefficient of linear thermal expansion different from that of the ceramic layer 53, and given that the electrode 54 is located near a top surface of the ceramic layer 53, the ceramic layer 53 having been formed by firing is susceptible to warpage. In addition to the warpage problem, the electrostatic chuck of FIG. 8 can also be adversely affected by a cooling gas leakage problem. Leakage of the cooling gas from the channel 55 to the outer periphery of the ceramic layer 53 is intended to be prevented by the sealed interface between the ceramic layer 53 and the semiconductor wafer W extending between the channel 55 and the outer periphery of the ceramic layer 53. However, if the distance between the channel 55 and the outer periphery of the ceramic layer 53 (sealed distance) is short (e.g., 1 mm to 2 mm) a gas leakage can occur through the sealed distance.
FIG. 9 is an illustration showing the electrostatic chuck of FIG. 8 with a modification to assist in preventing gas leakage through the sealed distance, in accordance with the prior art. In the electrostatic chuck of FIG. 9, the channel 55 is formed approximately 3 mm to 10 mm inside the outer periphery of the ceramic layer 53, thus providing a substantial sealed distance. Placing the channel 55 further from the outer periphery of the ceramic layer 53, however, requires the electrode 54 to be redefined to remain within the outer boundary represented by the channel 55, thus effectively decreasing an area of the electrode 54. Decreasing the area of the electrode 54 can cause insufficient electrostatic attraction in the region between the channel 55 and the outer periphery of the ceramic layer 53.
Furthermore, there are many examples of conventional electrostatic chucks in which a semiconductor wafer attracted to the chuck cannot always be readily detached, or “dechucked,” therefrom after a completion of etching or other processes. In some cases, it takes a considerable time for detachment of the semiconductor wafer.
In view of the foregoing, an apparatus is needed to overcome the problems associated with prior art electrostatic chuck arrangements. More specifically, the apparatus needs to prevent warpage of a ceramic layer and leakage of a cooling gas, while also enhancing electrostatic attraction and requiring only a short time for detachment.